1. Field of the Invention
The invention relates to a method for producing a semiconductor structure, in particular a semiconductor structure for optoelectronics.
2. Description of the Related Art
Owing to the lack of large-area monocrystalline substrates, high-power or optoelectronic components made of semiconductor materials with a wide bandgap (“wide-bandgap semiconductors”) such as GaN, AlGaN or InGaN—are nowadays often produced heteroepitaxially on suitable foreign substrates, for example monocrystalline sapphire or silicon carbide.
Such foreign substrates may, however, have properties detrimental to the function of the components produced on them. Some of these problems are known from C. A. Tran et al., Journal of Crystal Growth 298 (2007) 722. Sapphire has a relatively low thermal conductivity compared with other materials, the effect of which is that heat losses in high-power or optoelectronic components produced on it can be dissipated only poorly, which detrimentally affects the component function. It is furthermore known that LED structures, which are initially generated on sapphire but subsequently transferred onto a metal alloy substrate, have more favorable properties in respect of efficiency.
The use of silicon carbide (SiC) as a substrate resolves the heat loss problem owing to its high thermal conductivity. However, only few methods are available for generating SiC cost-effectively and on sizeable substrates. Bulk SiC substrates are available only up to a diameter of 100 mm.
U.S. Pat. No. 6,328,796 discloses an alternative method, in which a 3C—SiC layer is generated by carbonizing the surface of a silicon wafer which serves as a donor wafer, subsequently bonding the SiC layer of the silicon donor wafer onto a carrier wafer made of polycrystalline SiC, and then removing the rest of the silicon donor wafer in order to expose the SiC layer. Using polycrystalline SiC as a substrate has the advantage that, in contrast to silicon, the thermal expansion coefficients of GaN and SiC are relatively similar to one another. This can be advantageous for further heat treatment steps.
WO 03/034484 discloses another method with which, by means of ion beam synthesis (IBS), monocrystalline silicon carbide layers of the 3C polytype can be generated on silicon substrates (IBS 3C—SiC on silicon). In this method, the SiC layer is generated by implantation of carbon ions into a silicon substrate, heat treatment and removal of the parts of the silicon substrate which have been damaged by the ion implantation.
The surface of the SiC layer exposed in this way can be polished to a roughness of less than 0.5 nm by means of chemical-mechanical polishing, cf. EP 1 727 190 A1, so that GaN can subsequently be grown epitaxially.
WO 98/14986 discloses a method by which two material layers can be separated by exposing the interface of these material layers to electromagnetic radiation. The separation is based on the thermal decomposition of one of the two material layers, close to the interface. In practice, this method may be employed to produce optoelectronic components made of GaN or InGaN on sapphire, and subsequently separate the sapphire layer by thermal decomposition of the GaN or InGaN close to the interface between sapphire and GaN or InGaN.
A disadvantage with this laser lift-off method is the restriction to material systems which decompose under the effect of electromagnetic radiation or by the action of heat. This technologically complex method furthermore requires the use of intense lasers, with wavelengths which have energies exceeding the bandgap of the wide-bandgap semiconductors being used.